COIN/Harwell library issue

The configuration/build process for the Harwell library stub in COIN's ThirdParty?/HSL directory tries to create an empty library archive with 'ar cru .libs/libcoinhsl.a', however the ar included with Darwin (and I think other BSD-derived OSs too) doesn't support empty library creation in this manner.

I've worked around this by simply disabling the Harwell library configuration by passing COIN_SKIP_PROJECTS="Ipopt ThirdParty?/HSL" to Acro's configure. I included Ipopt since it had also been skipped by default, and I could not find an easy way to add additional packages to skip with overwriting the default setting of COIN_SKIP_PROJECTS. The latter might be a nice feature to have independet of this bug.